Not sure but could work.
Check /etc/make.conf if the ~x86 is there then it should work as I presume the ~ before the nero number means unstable ~amd or ~x86 same for 64bit.

Then just "emerge nero"
If you have the option set to stable amd / x86 and not to ~amd ~x86 then changing this ~ should work not sure how to make that option permanent or that emerge uses it as default ?

Could also use the
" USE="~x86" emerge nero "
replacing ~x86 with your cpu type. ~x86 , ~amd or similar.

Have to verify this info + how-to. Also not sure if nero is any good as several persons had bad luck in windows lately with nero , just use k3b